1. Field of Invention
This invention relates to framing synchronization in digital transmission systems and, in particular, to a novel technique of frame error detection and search control which allows a comparison of up to n bits on the occurrence of each framing clock pulse while an out-of-frame condition exists in a system which does not employ alternating framing bit sequences.
2. BACKGROUND DESCRIPTION
Most systems which have addressed the problem of reframing upon loss of synchronization have been concerned with framing patterns in which the framing pattern has been "winking", i.e., at the frame bit location the framing bit will alternately be a binary "1" and then a binary "0". One such system which employs an eight bit store and a frame control is described in U.S. Pat. No. 3,742,139, entitled "Framing System for T-Carrier Telephony," inventor M. A. Baehley. In this framing system, a group of eight bits are arbitrarily selected in a first frame and are fed to the store and compared with the corresponding time slots two frames later. Thus, the eight bits of the first frame are compared with the bits in the same time slot of the third frame, the third frame with the fifth, the fifth with the seventh and the seventh with the ninth, in each case with corresponding time slots. If framing is not achieved after the comparison between the seventh and ninth frames, the frame control circuit operates to delay the opening of the store by eight time slots, another series of comparisons is carried out, this time directed to a series of eight time slots immediately following the eight time slots in the first comparison. The process is repeated until framing is achieved.
Disadvantages of that technique include the circuit complexity and the necessity to wait through four frames before the frame shifting is initiated. An improved frame search control for a digital transmission system was disclosed in U.S. Pat. No. 4,203,003, issued May 13, 1980, for an invention entitled "Frame Search Control for Digital Transmission System," inventor Ralph L. Kline.
This technique was designed for use with a "winking" framing pattern, i.e., 101010 etc., rather than the more complex framing pattern of F1 F0 F0 F1 employed with the instant invention. Also, the monitor and search techniques of the instant invention were designed to respond more quickly as in desirable for use with a higher speed pulse train.